A de novo SCN8A heterozygous mutation in a child with epileptic encephalopathy: a case report

Background Epilepsy is a complex disorder caused by various factors, including genetic aberrance. Recent studies have identified an essential role of the sodium channel Nav1.6, encoded by the gene SCN8A, in epileptic encephalopathy. Case presentation Using parent-offspring trio targeted-exome sequencing, we identified a de novo heterozygous missense mutation c.3953A > G (p.N1318S) in SCN8A in a 3-year-and-9-month Chinese female patient with early infantile epileptic encephalopathy and a normal magnetic resonance imaging of the brain. Conclusions This de novo mutation was only detected in the patient but not in her parents. Bioinformatic analysis indicates the pathogenicity of this mutation. Administration of the sodium channel blocker well controlled seizures in the patient. Therefore, we recommend trio targeted-exome sequencing as a routine method for pathogenic variant screening in patients with intractable epilepsy and a normal MRI.


Background
Epilepsy is one of most common pediatric neurologic disorders. The prevalence rate is estimated 12/1000 in pediatric patients. One third of them shows pharmacoresistance, and 40% of patients who are younger than 3 years are related to the epileptic encephalopathy [1]. Epileptic encephalopathy (EE) refers to a heterogenous group of epileptic disorders, characterized by intractable seizure, impairment and regression of cognitive and behavioral functions [1,2]. The causes of EE include structure anomalies, inborn errors of metabolism and genetic insults. The exploration of numerous genetic variants is attributed to wide-spreading applications of nextgeneration sequencing [3], including ion channel mutations. Voltage-gated sodium channels are responsible for the initiation and propagation of action potentials. Malfunctions of sodium channels are involved in epileptic seizures [4]. Here we reported a patient with infantile EE probably caused by a de novo missense mutation of SCN8A (c.3953A > G, p.N1318S).

Case presentation
The 3-year-and-9-month old girl was born at term with uneventful maternal pregnancy, delivery and family history. Her birth weight and body length were both within normal limits. She had the first afebrile seizure at night sleep presented as generalized tonic-clonic seizure lasting for 1-2 min while she held the normal developemtal milestone of social smiling at her age of 2 months. Topiramate was prescribed, but seizures still happened, until oxcarbazepine monotherapy employed that seizure temporarily resolved for half a year. Later, afebrile seizure flared up and the frequency was around 1-2 fits per month, even several convulsive status epilepticus. She came to our attention 1 year later. Physical and neurologic examinations, including the head circumference, muscle tone and deep tendon reflex, and the laboratory data were all unremarkable. Metabolic survey of amino and organic acids, brain magnetic resonance imaging (MRI) and fluorodeoxyglucose (FDG)-positron emission tomography (PET) demonstrated unremarkable findings (Fig. 1). The test of Wechsler Preschool and Primary Scale of Intelligence (WPPSI-IV) showed mild cognitive impairment (full scale IQ = 66). Interictal electroencephalogram (EEG) revealed normal background activity without obvious epileptiform discharge.  Ictal video EEG recorded the seimology of generalized tonic, tonic-clonic seizures, and generalized electrical decrement with superimposed fast activity at EEG ictal onset (Fig. 2). Phenytoin (5 mg/kg/day), valproic acid (20 mg/kg/day), levetiracetam, clonazepam were ever tried, but there was little benefit. Meanwhile, short-term seizures that were provoked by low-grade fever developed then. Genetic counselling was recommended because her seizures were poorly controlled. High-throughput sequencing of exons of disease-causing genes were performed on the patient and her parents. Genomic DNA extraction and library preparation followed the standard Illumina protocols (Illumina, San Diego, USA) with minor adaptation provided by Joy Orient (Joy Orient Translational Medicine Research Center Co. Ltd., Beijing, China). Agilent Bioanalyzer 2100 (Agilent Technologies, USA) was used for quality control of DNA size distribution and enrichment. Target capturing was performed using Roche (Roche AG., Basel, Switzerland) product customized by Joy Orient, which uses 91,867 probes to capture total 7, 465,978 bp of exons regions of 3372 genes that are potentially associated with 4213 known Mendelian genetic diseases. A HiSeq2500 sequencer was used to perform high-throughput sequencing. Exon-enriched DNA was sequenced by the Illumina hiseq2500 platform following the manufacturer's instructions (Illumina). Raw image files were processed by the BclToFastq (Illumina) for base calling and generating the raw data. The low-quality variations were filtered out using the quality score ≥ 20 (Q20). The sequencing reads were aligned to the NCBI human reference genome version hg19 using BWA. Samtools and Pindel were used to screen SNP and indel of the sequence. All genetic variants were screened by pathogenicity, mode of inheritance and clinical phenotypes ( Table 1).
Importantly, a de novo heterozygous mutation c.3953A > G in SCN8A (the coverage of the variant (DP) is 174, allele frequency (AF) = 70/174 = 0.4) was identified and confirmed by Sanger Sequencing (Fig. 3a). The detailed information of this variant is as the followings: SCN8A (NM_014191.4), missense mutation, c.3953A > G (exon22), p.N1318S (de novo), location: chr12, 52,180,336. This missense mutation causes an amino acid substitution of an asparagine residue with a serine residue (p.N1318S) which occurs at a highly conserved LINKER position between the S4 and S5 segments in the third transmembrane domain (DIII) of SCN8A protein (Fig. 3b and c). This mutation site is absent from ClinVar [5] and HGMD Public [6]. It is neither recorded in the Exome Aggregation Consortium (ExAC) [7], which contains exome sequencing data from 60,706 unrelated individuals, nor in our 100 in-house controls. The substitution p.N1318S is predicted to be highly deleterious by bioinformatic tools, which predict possible impact of an amino acid substitution on the structure and function of a human protein, such as Polyphen2 [8] (HumDiv score = 1; HumVar score = 0.998), SIFT [9] (score = 0), Mutation Taster [10] (score = 1) and a comprehensive index CADD phred score (C score = 23.2) [11] (Table 2). Since the probable causative gene mutation was found, levetiracetam was stopped, but her family reported more seizures. Accordingly, the therapeutic regimen of a sodium channel blocker, lamotrigine (5 mg/ kg/day), valproic acid (24 mg/kg/day) and levetiracetam (10 mg/kg/day) were employed, then she had a second temporary seizure-resolved period of 5 months, even under high fever.
Discussion and conclusions SCN8A is widely expressed in the central and peripheral nerve systems during the neuronal maturation [4,12]. It is mapped to chromosome 12q13, encoding neuronal voltage-gated sodium channel α 8-subunit Nav1.6, which forms a complex combined with β subunits to modulate current conductance [4,13,14]. Nav1.6 consists of four transmembrane domains (DI-DIV), each containing six segments (S1-S6). Four S4 transmembrane segments are responsible for the voltage sense which contain positively charged arginine and histidine residues. There are fast and slow inactivation phases. Fast-inactivation phase is provided by internal DIII-DIV linkers to occlude the ion-conducting pores. Slow-inactivation phase is involved in a collapse of the pore, which is composed of S5-S6 segments of four domains [4].
Missense mutations of SCN8A accounting for 1% of EE are associated with a wide-spectrum phenotype of heterogenous epilepsy, and SCN8A missense  mutations are recently recognized to be associated with early infantile epileptic encephalopathy type 13, ref. [15][16][17][18] which displays multiple seizure types, including focal seizures, generalized seizures (tonic, myoclonic, absence) and epileptic spasms. These patients may experience stormy epilepsies [15,17]. Fever rarely triggers seizures [4,13,17]. Severity of psychomotor delay ranges widely after seizure onset [15,17]. MRI studies are typically normal [15]. There is no clear correlation between phenotypic severity and genetic mutations so far [1,4]. SCN8A mutations lead to premature channel opening, impaired inactivation and increased persistent current. Stormy seizures are reported resulting from gain-offunction effects [4]. It could explain that some patients respond well to sodium channel blockers in halting seizures [15,17,19,20]. As to our patient, she was categorized as the intermediate phenotype of EE, who manifested mild psychomotor retardation and infrequent seizure episodes, though developing stormy onset of generalized tonic and tonic-clonic seizures concomitant with normal EEG background activity [15,17,18]. Fever barely triggered seizures in our patient as previously reported [4,13,17]. MRI demonstrated no significant findings. She responds well to the sodium channel blockers. Targeted exome sequencing revealed a heterozygous missense mutation (c.3953A > G, p.N1318S) in SCN8A which is located in the internal S4-S5 linker of the DIII. This mutation could alter the function of the down-stream protein.
Since SCN8A (c.3953A > G, p.N1318S) is a de novo missense mutation, it is necessary to further validate its function in the future.